I've tried with different eselect-qtgraphicssystem settings and no change; having EGL enabled seems to be the problem. Other apps using OpenGL/EGL seem to work fine, including KWin. gdb spew attached, long version.
Created attachment 306795 [details] `gdb kdenlive` output
#0 0x00007ffff5551d60 in XVisualIDFromVisual () from /usr/lib64/libX11.so.6 #1 0x00007ffff35e48ee in qt_x11_getX11InfoForWindow (xinfo=0x2181e10, att=...) at kernel/qwidget_x11.cpp:3109 #2 0x00007ffff35e4990 in qt_x11_getX11InfoForWindow (xinfo=<optimized out>, a=...) at kernel/qwidget_x11.cpp:433 #3 0x00007ffff35e791c in QWidgetPrivate::create_sys (this=0x2181c30, window=50331711, initializeWindow=true, destroyOldWindow=176) #4 0x00007ffff3593f9a in QWidget::create (this=0xfc42b0, window=50331711, initializeWindow=true, destroyOldWindow=true) #5 0x00007ffff3c47d8d in QEgl::createSurface (device=0xfc42c0, config=0x197bb50, properties=0x0) at egl/qegl_x11.cpp:410
*** This bug has been marked as a duplicate of bug 405989 ***